(1) Field of the Invention
The present invention relates to a system and method for controlling a working hydraulic pressure (line pressure) in a mechanical power transmission with a fluid coupling device such as an automatic transmission for an automotive vehicle.
(2) Description of the Background Art
An engine output is input to a mechanical power transmission via a fluid transmitting device such as a torque converter or fluid coupling in an automotive automatic power transmission and no-shift power transmission. Particularly, in the mechanical power transmission device of the automatic transmission, up or downshifting is carried out by means of the engagement or release of a gear shift element such as a clutch or bandbrake.
It is noted that since a gear shift point is generally determined according to an opening angle of a throttle valve and vehicle speed and the engine operating condition is determined according to a kind of gear shift and opening angle of the throttle valve, a working hydraulic pressure is set, estimating the instantaneous engine generated torque, i.e., input torque toward the power transmission.
In addition, in order to improve a gear shift controlling accuracy, a hydraulic pressure controlling system has been proposed in which the hydraulic pressure is learned by detecting a target shifting duration determined according to the opening angle of the throttle valve.
However, since engine generated torque is varied according to the atmospheric pressure (altitude) or the engine state (i.e. warmed-up or not), it is, in some cases, difficult to estimate the engine generated torque on the basis of the opening angle of the throttle valve.
A Japanese Utility Model Registration Application First Publication No. Heisei 1-69947 published on May 10, 1989 exemplifies a line pressure controlling system for the automatic power transmission.
In the disclosed line pressure controlling system, an input revolutional speed and output revolutional speed of the power transmission are compared with each other to derive a speed ratio. An input torque of the mechanical power transmission is estimated on the basis of a characteristic value of a fluid transmitting device (torque converter) derived according to the speed ratio so that the line pressure is controlled on the basis of the estimated input torque.
However, since the fluid transmitting device may have various characteristics according to different types thereof as well as aging effects, it is difficult to accurately estimate the input torque, thus deteriorating the line pressure controlling accuracy.
Furthermore, although another line pressure controlling system in which a torque sensor is installed on an output axle of the power transmission to detect the output torque of the power transmission and the input torque is estimated on the basis of a gear ratio of the mechanical power transmission and the detected output torque, it is not cost effective and requires high precision. Consequently, such a line pressure controlling system as described above has not been put into practice.